1. Field of the Invention
The present invention relates to a novel process for the preparation of boron nitride.
2. Description of the Prior Art
Boron nitride is a known material that is increasingly in demand, in particular because of its stability at high temperatures, its resistance to thermal shock, its great chemical inertness and its very good heat conductivity. On the other hand, its low electrical conductivity makes it an insulator of choice.
Various processes for preparing boron nitride are presently known to this art.
One such process entails reacting boron trichloride with ammonia in the gaseous phase. A fine powder of boron nitride is thus obtained which can be sintered to produce solid shaped articles.
More recently, it has been discovered that it was also possible to prepare boron nitride by pyrolysis of polymer precursors.
One advantage of this polymer route resides especially in the possibility of shaping this type of product and, more particularly, for producing, after pyrolysis, fibers or films of boron nitride.
Thus, in U.S. Pat. No. 4,581,468 an organoboron polymer is described which is prepared by reacting ammonia (ammonolysis) with a trichlorotrialkylsilyl-borazole (cyclic compound) and which, after spinning and pyrolysis at a temperature of 970.degree. C., is converted into boron nitride fibers.
However, the cyclic starting compound described in the '468 patent is very difficult to prepare and thus is quite costly. Accordingly, such process is not applicable on an industrial scale.
On the other hand, the maximum yield by weight of boron nitride that can be produced from this type of product does not exceed 22%, which indicates, a priori, actual yields well below this value.
Finally, the ceramic material obtained after pyrolysis contains significant amounts of silicon because of the nature of the precursor itself.